Unless explicitly indicated herein, the materials described in this section are not admitted to be prior art.
There are numerous medical procedures that involve the insertion of a medical tool or instrument, such as a needle, cannula, catheter or stylet, into a subject's body, e.g. minimally-invasive surgical procedures, local anaesthesia, detection of bio-electrical signals, electrical stimulation for diagnosis or treatment, vascular access, fine needle aspiration, musculoskeletal injections and so on. In such procedures it is generally necessary to guide the medical tool properly to the desired position in the subject's body and it can also be beneficial to monitor or track the medical tool position to ensure that it remains at the desired location. In general it is very difficult for the user to determine the exact position of the tip of the medical tool and thus to be sure whether it is in the desired place, for example adjacent a nerve, or whether it has undesirably penetrated something else, for example a blood vessel.
It has been proposed to use x-ray techniques for needle guidance by providing the clinician with an x-ray image of the needle in the body. However in view of the risks associated with exposure to electromagnetic radiation, it is not possible to provide continuous guidance during insertion of the medical tool and so a series of snapshots are relied upon, which does not give optimal guidance.
More recently the use of ultrasound imaging to guide needle and catheterisation procedures has been proposed. Ultrasound imaging is advantageous compared to x-ray techniques because of the lack of exposure to electromagnetic radiation, and ultrasound probes are easily manipulable to image many different parts of the body. However ultrasound imaging has two main challenges: firstly that the interpretation of ultrasound images is rather difficult, and secondly that needles do not show-up particularly reliably or visibly in the ultrasound image.
As to the problem of needle visibility, the ultrasound image acquisition plane is thin—of the order of 1 mm thick, and so if the needle is out of that plane it will not be imaged. Further, even when the needle is in the imaging plane, because the echogenicity of standard needles is poor at high angles of incidence, the needle may not be particularly visible. It has been proposed to produce echogenic needles which make the needle more visible to ultrasound imaging devices. However these only help when the needle is well-aligned with the imaging plane. Similarly techniques for image processing and ultrasound beam steering help only when the needle is well-aligned with the imaging plane and do not work well for angles of incidence greater than 45 degrees.
Various needle tracking technologies have been proposed based either on a needle guide fitted to an ultrasound probe, e.g. U.S. Pat. No. 6,690,159 B2 or WO-A-2012/040077, or based on the transmission and reception of electromagnetic information, e.g. US-A-2007-027390), but these have functional and accuracy limitations which means that the needle tip position is not exactly known in every clinical circumstance. Typical accuracies are of the order of 2 mm, which can mean the difference between the needle tip being inside or outside a nerve. Further they often require the use of heavily modified or new equipment which is unwelcome to clinicians and to institutions with relatively rigid purchasing regimes.
Most often, therefore, practitioners rely on their skill and experience to judge where the tip of the medical instrument is as it is inserted. They may rely on sound, the touch and feel of the physical resistance to the medical tool and sudden changes in resistance, and changes in resistance to the injection of air or fluids. Developing this level of skill and experience is time-consuming and difficult and as there is an anatomical variation from patient to patient, the procedures inevitably entail some risks.
More recently it has been proposed to utilise magnetic tracking of a needle or other tissue-penetrating tool using a magnetometric detector attached to a freehand ultrasound probe and using a magnetised tissue-penetrating tool. Such a technique is described in our co-pending International patent application no. PCT/EP2011/065420. In this system a standard freehand ultrasound probe has a magnetometric detector attached to it, the detector comprising an array of magnetometric sensors. The sensors detect the magnetic field from the magnetised tissue-penetrating medical tool and send their readings of the magnetic field to a base station. The base station includes a data processor for calculating from the measurements the relative position and orientation of the tissue-penetrating medical tool relative to the ultrasound probe. The base station can supply this calculated position and orientation to the ultrasound imaging system so that the tissue-penetrating medical tool can be displayed on the ultrasound image of the subject's anatomy.
The system is advantageous in that it allows the operator to see both the ultrasound imaged anatomy and the magnetically detected tissue-penetrating medical tool on the same image. This enables greater accuracy in the procedure. Further, the attachment of a magnetometric detector to the ultrasound probe does not alter the feel of the ultrasound probe significantly, and it remains, therefore, familiar to the practitioner. Similarly the magnetization of the tissue-penetrating medical tool does not alter its physical characteristics, again, preserving the familiarity and experience of the clinician. The system is also simple and cheap compared to optical or electromagnetic tracking technologies and because the ultrasound probe can be manipulated freely, the ease-of-use of the freehand ultrasound system is preserved.
The system requires, however, that the tissue-penetrating medical tool is reliably and consistently magnetised.
Accordingly the present invention provides a device and method for magnetising a tissue-penetrating medical tool. In particular at least part of the tissue-penetrating medical tool is magnetically saturated by the magnetization device and method. The device and method preserve the sterility of the tool while reliably magnetising the tool to the extent necessary. The device and method may also be adapted to magnetise a defined length of the tissue-penetrating medical tool.
In more detail one embodiment of the invention provides a device for magnetizing a tissue-penetrating medical tool comprising a tool-receiving space for receiving at least part of the tissue-penetrating medical tool; a magnetic flux generator generating a magnetic field, the magnetic field having a magnetization region for magnetically-saturating the part of the tissue-penetrating medical tool which is in said tool-receiving space, the magnetic flux in the magnetization region being oriented in a direction substantially parallel to a longitudinal axis of the tissue-penetrating medical tool.
Another aspect of the invention provides a method of magnetising a tissue-penetrating medical tool comprising: positioning at least part of the tissue-penetrating medical tool in a tool-receiving space; generating a magnetic field in said tool-receiving space to magnetically-saturate the part of the tissue-penetrating medical tool which is in said tool-receiving space, the magnetic flux in the magnetization region being oriented in a direction substantially parallel to a longitudinal axis of the tissue-penetrating medical tool.
Preferably the tool receiving space is adapted to permit movement of the tissue-penetrating medical tool in a movement direction parallel to the longitudinal axis of the tissue-penetrating medical tool. Preferably the tool-receiving space is adapted to admit a predefined length of the tissue-penetrating medical tool, and more preferably to allow it to be moved into and out of the tool-receiving space in opposing movement directions. The tool-receiving space may have a longitudinal axis substantially parallel to the longitudinal axis of the tool and substantially parallel to the magnetic flux in the magnetisation region.
The magnetic flux generator may be provided on one side of the tool-receiving space. More preferably the magnetic flux generator is provided on two sides of the tool-receiving space. Alternatively the magnetic flux generator may surround the tool-receiving space, e.g. by having a cylindrical configuration.
The magnetic flux generator may comprise a stationary part and a movable part, the stationary part generating a magnetic field extending through the magnetization region and the moveable part being movable towards and away from the magnetization space so that its magnetic field is selectively applied to the magnetization region. The movable part of the magnetic flux generator may comprise a plurality of magnets positioned along a direction parallel to the longitudinal axis of the tissue-penetrating medical tool. The plurality of magnets may have alternating pole orientations. Preferably the plurality of magnets comprise a first set of magnets with alternating poles on one side of the magnetization region and a second set of magnets on the opposite side of the magnetization region, the second set of magnets having the same pole orientations as the first set.
Preferably the movable part of the magnetic flux generator is movable towards and away from the magnetization region in a direction transverse to the longitudinal axis of the tissue-penetrating medical tool.
The tool receiving space may be constituted by a longitudinally-extending space.
The magnetic flux generator may be a permanent magnet or electromagnet.
A conveyor belt may be provided to convey a tissue-penetrating medical tool through the magnetization region in the tool-receiving space. Where an electromagnetic and conveyor belt are used together, the electromagnetic may be controlled to vary the strength and/or direction of the magnetic flux in the magnetization region as the tissue-penetrating tool passes through the magnetization region. Preferably an optical sensor is provided to detect the position of the tissue-penetrating tool as it passes through the magnetization region.
The tool-receiving space may have one open end for receiving the tool and a closed end, the length of the tool-receiving space thus defining a length of tissue-penetrating medical tool which is within the magnetization region.
The tool-receiving space may comprise a sterile liner such as a disposable drape and/or disposable plastics tube. The disposable plastics tube may be a standard needle or cannula cover.
In one embodiment the device is sterile and, optionally, disposable.
The device is preferably hand-held and optionally is provided with a guard extending around the entrance to the tool-receiving space to protect the user's hand. The guard may be a plastics shroud or protective lip.
The tissue-penetrating medical tool can be a needle, cannula, stylet, or the like.